Conventionally, research has been conducted on cosmetics that make the skin firm and elastic. As the cosmetics that make the skin firm and elastic, cosmetics that are formulated with a moisturizing component having excellent water retention and excellent water absorbency, such as various water-soluble polyhydric alcohols, mucopolysaccharides, and polymers, and cosmetics that are formulated with an active ingredient for making the skin firm and elastic such as plant extracts are commercially available. Cosmetics that are formulated with these components are improved in the effect of making the skin firm and elastic; however, such cosmetics have problems of having less preferable feel in use, e.g., poor permeation into the skin or lingering stickiness. A cosmetic that is formulated with glycerin or other polyhydric alcohols is known to have a problem of causing stickiness that is likely to linger on the skin (Patent Document 1).
In order to solve these conventional problems, various cosmetics have been developed by considering the types and the formulation amounts of moisturizing components to be formulated. For example, a method for formulating a polymer having a basic amino acid residue in the structure and a macromolecule (xanthan gum) in combination (Patent Document 2), a method for formulating raffinose and agar-agar in combination (Patent Document 3), a method for formulating glycol and an adduct of diglycerin with propylene oxide in combination (Patent Document 4), and the like have been developed. Besides, a method for formulating a component that has an effect of inhibiting a collagenase, which is one of the causes of making the skin less firm and less elastic (Patent Document 5) and a method for formulating dimer linoleyl bis(phytosteryl/behenyl/isostearyl) dimer dilinoleate and gellan gum in combination at a certain amount (Patent Document 6) have also been developed. However, these methods cannot yet produce a cosmetic or an external skin preparation that has excellent texture in use, e.g., no crinkles of a cosmetic or smooth application.
A skin cosmetic (eye cream) that is prepared using a water dispersion of urethane resin that is synthesized by a reaction of a polyol compound and an isocyanate compound is recently reported. The use of the skin cosmetic is reported to give a feeling of improved wrinkles on the skin and a feeling of improved sagging (a feeling of firmer skin), and to have an improved feel in use, e.g., less stickiness, less crinkles of a cosmetic on the skin, and less unsmooth application (Patent Document 7). However, little studies have been conducted on this type of cosmetics other than eye creams regarding the feel in use, e.g., crinkles or unsmooth application. Currently, development of a cosmetic and an external skin preparation is longed for that is excellent in permeation into the skin, is excellent in stretching on the skin, and has no unpleasant feel in use, e.g., stickiness, crinkles on the skin, or unsmooth application.
Polymer gel (chemical gel and physical gel) that is obtained by crosslinking of macromolecular compounds via a covalent bond, an ionic bond, a hydrogen bond, or the like is known. It is known that the more polymer gel a product contains, the more viscous the product is, and as a result, dripping is less likely to occur, rubbing the product into fingers is easier, and the scattering of the product when rubbed is prevented. However, it has been pointed out that high polymer gel content causes stickiness (Patent Documents 8 and 9) and causes crinkles when drying (Patent Document 10). Conversely, a cosmetic that contains less polymer gel is less likely to cause stickiness and crinkles, but it is more prone to drip and is less easily rubbed into fingers. Enough studies have not yet been conducted on the feel in use, e.g., stickiness and crinkles on the skin, and therefore a method that can collectively solve dripping, stickiness, and crinkles is desired.
Meanwhile, interest in anti-aging technologies and tissue engineering has grown rapidly, and development of a cosmetic that is highly biocompatible and highly safe has been desired. In recent years, hydrogels are used in various fields as a highly biocompatible material having water as a medium, and most of the hydrogels are a hydrogel that includes a macromolecular compound or a hydrogel that is formulated with an inorganic material, namely a hydrophobized powder. When these hydrogels are used in a cosmetic or an external skin preparation, however, these hydrogels have problems of an unreacted substance resulting from the synthesis of the macromolecular compound and of the safety of the inorganic material itself.
In recent years, active research on a low-molecular hydrogelator containing a low-molecular compound has been conducted, because the functions of the low-molecular hydrogelator have attracted a lot of interest despite the fact that determination of a mechanism of self-organization among low-molecular compounds in water and molecular design are difficult. As a result, some low-molecular hydrogelators have been discovered (Non-patent Documents 1 and 2). Most of them are amphiphilic compounds that have both a long-chain alkyl group as a hydrophobic moiety and a hydrophilic moiety, and examples of these include ones having an amino acid (Non-patent Document 3), ones having a peptide (Patent Documents 11 and 12), ones having a mono- or polysaccharide (Non-patent Documents 4, 5, and 6), and ones having a polyol (Non-patent Document 7), as the hydrophilic moiety. A low-molecular gelator has been developed utilizing the fact that a peptide including valine easily assumes a β-sheet structure (Non-patent Document 8).
Low-molecular hydrogelators that cannot gel water alone nor organic solvent alone but gel aqueous alcohol solutions and/or organic solvent aqueous solutions have been reported. A well-known common characteristic of low-molecular gel is to quickly react to external stress to convert from gel to sol.